SatNav thro Car speakers and listening to music CD

Is it possible to set the output of a SatNav Nuvi 765T to the car speakers (via FM transmitter) and then listen to a normal CD on the car stereo..... and then when the SatNav outputs some audio directions, that they 'cut through' the music like a Traffic message ?

Or does one have to tune the car stereo into the required FM frequency.... and have the car stereo purely dedicated to the SatNav ? No music on the car stereo while using the SatNav output to the car speakers?

Thanks,
Dave